AQP4 and neuromyelitis optica: According to their AQP4-IgG serostatus, we could classify definite and high risk NMO patients into three different groups: AQP4-IgG seronegative patients, patients with Abs against M-23 AQP4 but not against full length AQP4 and patients with Abs against both AQP4 isoforms.